The present application is directed to a method of controlling the speed of an aircraft, and more particularly to a method for controlling the speed of an aircraft having a preprogrammed speed profile when transitioning from a manually set target speed to the preprogrammed speed profile.
Modern jet transports are equipped with a cockpit mode control panel that interfaces with a flight management system to control the selection and engagement of automatic flight control modes of operation. These automatic flight control modes of operation include, for example, flight level change (FLCH), vertical navigation (VNAV) and lateral navigation (LNAV). The FLCH mode can automatically manage thrust and speed to climb or descend from one altitude to another. The VNAV mode can provide automatic optimized profile control from initial climb through final approach, including adherence to terminal area procedure speed and altitude constraints. The LNAV mode can provide steering to a preprogrammed route including selected terminal area procedures.
The pilot chooses the available modes that will best accomplish the desired vertical flight profile and lateral routing. In most instances, the pilot plans the flight in advance, both laterally and vertically, and preprograms the LNAV and VNAV modes so that the desired flight path will be followed. While preprogrammed flights are advantageous because they reduce the pilot's burden, particularly during takeoff and landing, in practice, rarely can flights be flown as preplanned. For example, rerouting and clearance instructions may be received from air traffic control (ATC) during the flight. These instructions force the pilot to depart from the vertical flight profile and/or the lateral route that was originally planned. In some instances, rerouting and reclearance come far enough in advance to allow the pilot to reprogram the route or profile instructions stored in the memory of a flight management computer so that the flight management system can remain in the LNAV and VNAV flight control modes. On other occasions, pilots are forced to manually intervene in order to depart from LNAV and VNAV preprogrammed flight paths and comply with ATC instructions in a timely manner.
Intervention-capable flight management systems (FMS) have been developed which allow a pilot to intervene in the operation of the preprogrammed flight management computer of a flight management system and change the speed and/or flight path of an aircraft in response to air traffic control (ATC) instructions. One such system is disclosed in U.S. Pat. No. 4,811,230, issued to Graham on Mar. 7, 1989 and entitled “Intervention Flight Management System,” the disclosure of which is hereby incorporated by reference in its entirety.
The intervention FMS disclosed in the Graham patent includes a mode control panel via which the pilot interfaces with an FMS program. The FMS program includes several modules that override the preprogrammed instructions stored in the memory of the flight management computer when the modules are engaged. In this manner, the FMS allows the pilot to manually intervene and control the flight management computer and, thus, the aircraft in response to, for example, ATC instructions to change heading, altitude, airspeed or vertical speed. The FMS automatically returns to fully optimized flight along the preprogrammed profile when the intervention is cancelled.
Under certain conditions, returning to the preprogrammed profile after the intervention is cancelled has been known to cause problems. For example, when in descent, ATC instructions often require a slower speed than the VNAV preprogrammed profile speed, causing the pilot to intervene using the FMS to manually set a slower target speed. Upon completing the ATC procedure, the pilot may wish to exit speed intervention, and return to the VNAV preprogrammed profile. In this scenario, it is often the case that the scheduled VNAV profile speed remains higher than the manually set target speed. If so, the flight control computer will signal an increase in thrust from the engines, causing an increase in the speed of the aircraft as the computer attempts to return the aircraft to the scheduled VNAV profile speed. This increase in speed is generally undesirable during descent, when a reduction in speed is typically necessary in order to eventually reach the desired landing speed. The speed increase wastes fuel, can cause potential safety issues, and can result in additional work for the pilot, who may attempt to manually adjust the throttle of the aircraft to avoid the speed increase.